PROJECT SUMMARY Autism spectrum disorder (ASD) is characterized by core social impairments which limit patients? ability to form and maintain meaningful relationships. At present, antipsychotics are the only medication approved to treat ASD, but they target associated symptoms, have unfavorable side-effects, and do not treat ASD?s core social deficits. Developing new medications that specifically target social functioning will thus address an important unmet need. A large body of research has shown that the neuropeptide arginine vasopressin (AVP) plays a critical role in promoting social behavior and that experimental dysregulation of the AVP signaling pathway produces social deficits in animal models. Although intranasal AVP administration improves social cognition and memory in neurotypical individuals, no published research has tested the effects of AVP treatment in ASD patients. Several lines of evidence underscore the necessity of such research. For example, we recently reported that blood AVP levels predict theory of mind ability in children with ASD, such that children with the lowest AVP levels have the most marked theory of mind deficits. This finding is consistent with our preclinical research showing that socially impaired monkeys have significantly diminished cerebrospinal fluid AVP levels compared to control monkeys. Similarly, data from the first neuropeptide receptor mapping study of postmortem primate brain tissue revealed that AVP V1a receptors are widely distributed throughout the extended neural amygdala pathway, suggesting that AVP administration can target directly neural pathways known to regulate social functioning. Interestingly, AVP?s pharmacological effects are especially evident in male animals, and given ASD?s male-biased prevalence, AVP deficits may be particularly relevant to understanding the risk for, and treatment of, ASD. We recently tested the effects of 4-week intranasal AVP treatment in children with ASD in a double-blind randomized placebo- controlled pilot trial (R21 MH100387; MPI: Parker & Hardan). AVP was overall well tolerated in this small sample, and importantly, AVP treatment improved social abilities in children with ASD as assessed by parent ratings on the Social Responsiveness Scale, 2nd Ed (SRS-2). This result was more pronounced when we accounted for pre-treatment blood AVP levels. Here we seek to extend these findings in a larger ASD study cohort (N=100), aged 6 to 17 years, in this double-blind randomized placebo-controlled 8-week trial. Our primary outcome measure is improvement in child social abilities as assessed by parent ratings on the SRS-2. We will also test the safety and tolerability of AVP treatment, and whether pre-treatment blood AVP levels are a personalized predictor of treatment efficacy. Finally, we will test whether AVP treatment improves ASD symptoms as assessed by clinician impression, additional parent report measures, and child performance on laboratory tests of social cognition and communication. We predict that AVP treatment will improve social abilities in children with ASD, and that AVP will be well tolerated, in keeping with our preliminary data. This research has high potential to lead to development of the first effective medication to treat ASD?s currently intractable social deficits.